In a UMTS (Universal Mobile Telecommunications System) network, for the purposes of achieving higher-speed data rates, providing low delay and so on, long-term evolution (LTE) has been standardized (see Non Patent Literature 1). In LTE, as multi access schemes, an OFDMA (Orthogonal Frequency Division Multiple Access)-based scheme is used for downlink and an SC-FDMA (Single Carrier Frequency Division Multiple Access)-based scheme is used for uplink. For the purposes of achieving further broadbandization and higher speed, successor systems to LTE have been also studied and standardized (Rel. 10/11) (for example, such a system is also called “LTE advanced” or “LTE enhancement” (hereinafter referred to as “LTE-A”)).
Further, in a future radio communication system (Rel. 12 or later), the LTE system has been considered as operating not only in a frequency band (licensed band) that is licensed to a communication carrier (operator), but also in a license-not-required frequency band (unlicensed band) (which system is called LTE-U: LTE Unlicensed). In LTE-U operation, if the LTE-U is operated on the basis of cooperation of the licensed band LTE (Licensed LTE), it is called LAA (Licensed-Assisted Access) or LAA-LTE. A system operating LTE/LTE-A in the unlicensed band is sometimes called LAA, LAA-LTE, LTE-U, U-LTE collectively.
The licensed band is a band that is permitted to be used exclusively by a specific operator (carrier), while the unlicensed band (also called “non-licensed band”) is a band in which a radio base station is able to be installed without restriction to a specific operator. As the unlicensed band, a 2.4 GHz or 5 GHz band, in which Wi-Fi (registered trademark) and Bluetooth (registered trademark) are available, and a 60 GHz band, in which milli-meter wave is available, have been considered to be used. This unlicensed band has been also considered to be applied to a small cell.